1. Field of the Invention
The present invention relates generally to a distributed process control system for controlling the operation of an industrial process and, more particularly, to such a system which is workstation or personal computer-based and utilizes real time, bumpless transfer of control between running and backup workstations to achieve full process control system redundancy.
2. Description of the Relevant Art
Over the years process control systems have been enhanced and new systems developed to accommodate the ever increasingly sophisticated control demands of plant operations. For example, process control systems have been developed or enhanced to safely, efficiently and economically provide automated control for advanced steel manufacturing, chemical processing, power generation, water treatment applications and the like. A valuable feature of each of these process control systems is the capability for system expansion so that the control system may "grow" as the process being controlled grows, while at the same time not limiting the nature of the control processes available. Another valuable feature of these process control systems is that they are adaptable to a wide variety of industrial operations without requiring hardware modifications, i.e., they may be reconfigured by performing software modifications.
One of the earlier process control systems utilizes a central or "host" computer interfaced to local data acquisition and control devices. All decisions regarding the operation of the controlled process were made by the host computer. Although this system operated with some success, it had some major drawbacks. For example, utilizing a host computer in a process control system requires that the control system be predefined prior to its installation so that the precise arrangement of, for example, the communications between the host computer and the various processes controlled can be defined. Further, in such a process control system, a malfunction occurring within the host computer could adversely effect the operation of the entire control system and possibly cause the entire control system to fail.
In recognition of these drawbacks, process control systems were developed that moved away from the central/host computer configuration to a "distributed" control arrangement. In the distributed process control arrangement, the intelligence necessary to gather data and to take appropriate control action in response to such data is located in local "drops" distributed throughout the process which it controls so that central computers and the like are eliminated. In these distributed systems, all drops are connected by a single data highway. Each drop has allocated to it a specified time period during which it outputs various parameters it generates to the highway for acquisition by other drops, and another time period is provided in which all the drops may output other non-recurring data. Since this distributed control arrangement is modular in nature, it allows for the growth of the control system simply by adding new drops and further simplifies drop modifications required to meet process changes.
Although the distributed process control system approach provides an advancement in the art over control systems utilizing a host computer, the distributed process control system approach does, for some potential users, introduce control system limitations. For example, since all distributed process control systems and process control networks presently employ proprietary data processing hardware and associated software for actual process control (proprietary in the sense that such hardware and software are available only from the distributed process control system/vendor/provider), purchasers of such available distributed process control systems are prevented from using "off the shelf" or non-proprietary data processing hardware and other non-proprietary computer programs in conjunction with data processing hardware to effect process control. In addition, this same hardware is not typically used to perform other non-control operations such as data acquisition, data recording and archiving, spread sheet preparation, special process calculations and process control system modifications. As a result, additional, complementary system hardware is required to allow the user to view the process or to perform these other control system functions. Although the requirement of additional system hardware does not in all instances create a problem, for some applications the inclusion of this additional hardware increases the cost of the system to a level which makes the use of the system itself impractical.
As seen from the foregoing, presently available process control systems which utilize either a central or host computer or associated distributed process control arrangements are not without their shortcomings for some applications. Consequently, there is a need for an improved distributed process control system which may be used to control the operation of an industrial process which overcomes these shortcomings.